Expanding the horizon of molecular electronics via nanoparticle assemblies

A Clever Combination :
Organic Molecules &
Nanoparticles
In view of current and future technological developments, Si-based mi-
croelectronics faces a constant need for miniaturisation. However, its
evolution towards nanoelectronics presents severe physical and economic
limitations. Several potential alternatives to supplement or to replace the
current technology have been investigated, like silicon MOSFETs1,2,3, or
techniques based on novel materials such as carbon nanotubes4,5. An-
other alternative is based on the use of single organic molecules, acting
as electronic switches and storage elements6. Indeed, molecules exhibit
inherent advantages over the current devices, making them appear as the
ideal object for designing future high-density electronic devices. They are
several orders of magnitude smaller than present feature sizes. They may
be produced in large amounts absolutely identically and in a cheap way by
chemical synthesis. Their physical properties are tunable by their struc-
tures. In addition, they have the potential to self-organise into regular 2D
or 3D patterns. Therefore, complete systems for information processing
may be built from basic functional units consisting of molecules acting as
logic devices.
The use of molecules to perform electronic functions was first pro-
posed in 1971 by Kuhn 7, followed by Aviram and Ratner 8 in 1974, who,
based on theoretical assumptions, envisioned the use of a donor-acceptor
molecule to produce a molecular rectifier. At this time, any realisation of
a molecular device was technologically absolutely unfeasible. Nowadays,
the development of this visionary concept and its extension into the ex-
perimental domain has become the broad research area called Molecular
Electronics 9, as demonstrated by the increasing number of citations of
this early paper (Figure 1.1). The real beauty of this concept is the per-
spective that specific electronic functions of a device may be adjusted
by the design of the chemical structure of the molecules10,11. Moreover,
additional tunability by other stimuli, like voltage, light or magnetic field
can be envisioned.
In this research area, two very di®erent approaches are under exten-
sive investigation: the bulk molecular systems and the single molecular
systems. The first one is defined by characteristic dimensions which are
much larger than the sizes of the molecules. Consequently, most of the
molecules are in contact with other molecules, making them not individ-
ually addressable. The properties of this ensemble must be considered as
a whole. In contrast, single molecular systems aim for individual contact
to single molecules or small arrays of perfectly ordered molecules. This
second approach strongly di®ers from the first one, as it tries to utilise the
physical properties of single molecules for nanosized electronic devices.
A basic requirement for Molecular Electronics is the connection of
the molecules to the outside world. In the case of bulk molecular systems,
this is achievable thanks to conventional UV or electron-beam lithogra-
phy, which enables the fabrication of micrometre-scale contact patterns
(down to 20 nm). For single molecular systems, an electrode pair with
nanometre-sized spacing to contact a single molecule is needed. Although
the development of such a device is challenging, the technological advance
would provide the possibility to address one or few molecules. Several
techniques were investigated, such as STM12,13,14, mechanically control-
lable break junctions (MCBJ)15,16, cross-bar arrays17,18,19, electromigra-
tion20,21 or mercury droplet22. Quantum phenomena such as Coulomb
blockade20, Kondo e®ect21 and negative di®erential conductance10 have
already been observed. However, reproducibility and stability of such re-
sults still remain uncertain23. Indeed, the mobility of electrodes' atoms
makes the metal-molecule junction unstable, strongly a®ecting the prop-
erties of the device, meaning that large ensembles of molecules would
therefore be more suitable for technological applications. Devices devel-
oped using this second approach are already present on the market, such
as for example liquid crystal displays or organic light emitting diodes.
Although tremendous progress was achieved on both approaches over
the past few decades24,25,26, the understanding of current transport through
molecules remains uncomplete. Both experimental and theoretical evi-
dence show that the electronic properties of molecular junctions are not
only dictated by the molecules contacted, but also depend on the anchor-
ing groups and on the electrodes forming the junction23,27. Thus, along
with the properties of the molecule itself, those of the block \contact-
molecule-contact" also have to be considered. Introducing such building
blocks into functional electronic circuits remains a demanding task, re-
quiring innovative approaches in fabrication philosophy and circuit struc-
ture17,28. Nanometre-size metallic nanoparticles as intermediate elec-
trodes can be used as an elegant solution to build molecular junctions
with well-defined geometry and electronic properties. Indeed, combin-
ing metallic nanoparticles with molecules to form such building blocks
would overcome the size mismatch between nanometre-scale molecules
and micrometre-scale electrodes29,30,31,32,33,34,35.
We propose in this PhD Thesis a multidisciplinary study, interlink-
ing chemistry (synthesis of nano-objects constituting building blocks),
engineering (design of circuits based on the assembly of such building
blocks) and physics (understanding of the ensemble properties). Con-
cretely, two projects based on the assembly of nanoparticles were per-
formed, on one hand, one-dimensional assembly, and on the other hand,
two-dimensional assembly. The first one combines nanolithography and
electrostatic trapping of colloids to achieve the fabrication of conducting
nanoparticle chains. The second project consists of the self-assembly and
the micro-contact printing of ligand-protected nanoparticles, followed by
in-situ ligand exchange reactions. This fabrication method enables the
preparation of stable two-dimensional networks of molecular junctions.
Remarkably, this approach, combining the single molecular approach and
the bulk molecular approach introduced above, takes the advantages of
addressability of the first one and stability of the second one.